Fire-Driven Thunderstorms

In 2016 and 2017, wildfires in western Canada spawned thunderstorms
that ignited additional fires, in some cases tens of miles away from the
original fire. These fire-triggered
thunderstorms are technically known as pyrocumulonimbus clouds, or “pyroCb’s”.

The physics of pyroCb’s is complex.
When super-heated updrafts from an intense fire suck smoke, ash, burning
materials, and water vapor high into the air, these elements cool and form
so-called fire clouds that look and act like the cumulonimbus clouds associated
with classic thunderstorms. What is
different is that the heat and particulates in the smoke almost always arrest
the ability of the cloud to produce rain.
Instead, what remains is a lightning storm that moves across the
landscape, triggering more fires.

These PyroCb events appear to be happening far more often, producing
more energy, and erupting in places where they have never been seen
before. As the world warms, wildfires themselves
are becoming larger and hotter. In the
past decade, wildfires have been burning more than twice as many acres as they
did before the turn of the 21st century. Along with the growth in wildfire activity,
there has been an increase in PyroCb events, and there are now an average of 25
per year in western North America.

Apart from starting new fires, pyroCb’s also have similar effects as
moderate-sized volcanic eruptions. Smoke
and aerosols from wildfires can rise high into the stratosphere, where they can
linger for months. Eventually, the
particles carried aloft in the atmosphere do come down, dumping dangerous
chemicals on far flung regions of the earth.
But unlike volcanic eruptions, which are relatively rare events, pyroCb’s
are happening more and more each year.